`pw_stream_destroy()` chains up to `on_remove_buffer()` in
GstPipeWireSrc which again needs the stream to still be around to call
`pw_stream_queue_buffer()` on it. By using `g_clear_pointer()` it will
already have been cleared, causing crashes.
Revert to the behavior from before the commit mentioned below and add a
comment in order to avoid regressing in a future cleanup.
Fixes: 0c40c0147 (gst: factor out the stream management and some common variables in a new class)
Construct this new class from both the src and sink to be able to share code
Consolidate the previous mess of open/close/start/stop into a single pair
of open/close functions in the new stream class
